1. Field of the Invention
The present invention concerns an automatic valve for controlling the flow of fluid and, more specifically, it relates to an automatic valve for controlling the flow of fluid which valve is adapted to conduct ON-OFF operations utilizing the magnetic attraction force between permanent magnets.
2. Description of the Prior Art
Automatic valves of the afore-mentioned type have been disclosed, for example, in Japanese Utility Model Application Laid-Open Sho 60-8582. Referring to the descriptions in the specification and the drawings (the outline of which is illustrated in FIG. 6 of the drawings, appended hereto only for the reference), a valve shaft 4 that has a valve body 2 formed at the top end thereof for closing a fluid port 1A and a multi-layered cylindrical magnet MS of a small diameter comprising a plurality of ring magnets 3, 3,--stacked axially on the shaft and mounted on the rear end thereof, are housed within a valve chamber 5 so as to be axially slidable relative to the valve chamber.
At the outside of the valve chamber 5, a multi-layered cylindrical magnet ML of a large diameter comprising a plurality of axially stacked ring magnets 6, 6--is disposed coaxially with the cylindrical magnet MS and slidably along the axial direction of the valve shaft 4. In the stack of the magnets MS and ML, the magnetic polarity (N or S) is made different between each of ring magnet pair 3 and 6 radially opposed to each other. A piston 7 having the cylindrical magnet ML is disposed within a cylinder 8 and adapted to slide in a direction for opening or closing the valve by a pressurized air or gas. The cylinder 8 has ports 9A and 9B for the pressurized air or gas, formed at both of the top and rear ends thereof on both sides of the piston 7, so that the piston 7 is driven by the pressure of the pressurized air or gas supplied alternately from the ports 9A and 9B.
When the piston 7 is caused to slide in the direction of opening or closing the valve 7, the valve shaft 4 mounted with the multi-layered cylindrical magnet MS is caused to slide forwardly and backwardly by the magnetic attraction force of the cylindrical multi-layered magnet ML, as a part of the piston 7, to actuate the valve body 2, thereby opening or closing the fluid port 1A.
Since the valve shaft 4 housed within the valve chamber 5 is adapted to move slidingly without protruding to the outside through the side wall of the valve chamber 5, thereby actuating the valve body 2 attached at the top end of the shaft 4, a tight sealing can be obtained in the valve chamber 5 formed between the two ports 1A and 1B.
However, in a case where the automatic valve of such a structure is used, for example, as a color-change valve in a multi-color coating apparatus, there are various problems.
Firstly, the valve body 2 when put in a closed state, may possibly be separated from the valve seat 10 under the effect of back pressure of paint acting from the side of the port 1A.
Specifically, the valve of FIG. 6, when used as a paint color-change valve is connected, for example, at the port 1B, to a paint supply source of a certain color and connected at the other port 1A, by way of a paint hose, to a manifold connected to a coating machine. However, since other color-change valves for supplying paints of other different colors are also connected with the manifold, if the magnetic attraction force between the cylindrical magnets MS and ML for closing the color-change valve is not strong enough, the valve body 2 for closing the port 1A may possibly be separated from the valve seat 10 under the effect of the pressure of a paint of different color supplied from other color-change valve to the inside of the manifold. This may bring about a risk of defective coatings due to the intrusion of the paint of different color into the valve chamber 5 and result in mixing of these paints
Secondly, in the conventional automatic valve shown in FIG. 6, since the piston 7 is adapted to be driven by alternately supplying pressurized air or gas (simply referred to as air, hereinafter) from the ports 9A and 9B formed on both of the top and rear ends of the cylinder 8, air hoses have to be connected to both of the ports 9A and 9B.
When the conventional automatic valve is used as a color-change valve in a multi-color coating apparatus for conducting coating under color-change, e.g., for more than thirty kinds of colors, since a great number of air hoses have to be disposed in addition to a number of paint hoses, the number of pipeways, as well as switching valves for selectively supplying pressurized air to be respective air hoses is also increased, so as to necessitate the expenditure of a huge installation cost and require a larger installation space.
Furthermore, when the piston 7 is driven by the pressurized air supplied from one of the ports 9A or 9B, since the air pressurized by the piston 7 in the cylinder 8 has to be discharged from the other of the ports 9B or 9A, the ON-OFF control for the switching valve disposed to each of the air hoses becomes extremely complicated.
In order to overcome such a problem, the present inventors have attempted to put the piston 7 under the resiliency of a spring that normally closes the valve and drive the piston in the valve opening direction by the pressurized air supplied from the opposite direction and, at the same time, release the air pressurized in the cylinder 8 upon driving the piston 7 directly to the outside through a discharge hole perforated to the circumference of the cylinder 8.
However, in the automatic valve used for the color-change valve, since a solvent such as a thinner is blown to the surface thereof for cleaning and removing a paint which is sprayed from the coating machine and deposited thereon, the solvent may possibly intrude through the hole perforated in the circumference of the cylinder 8 to the inside thereof, thereby deteriorating the sealing members such as rubber packings, etc. disposed on the inside of the cylinder.
Thirdly, when the automatic valve illustrated in FIG. 6 is used as a paint supply valve for controlling the flow of paint supplied to a coating machine, the paint remaining in the valve chamber 5 can not completely be removed upon cleaning of paint pipeways in which the valve is disposed.
Specifically, if a cleaning solvent is supplied, for example, from the port 1B into the valve chamber 5 and discharged from another port 1A, it is impossible to completely remove the paint that has intruded into narrow gaps formed between the inner circumferential surface of the cylindrical magnet MS that slides integrally with the valve shaft 4 or the paint that has further invaded as far as the rear end of the valve shaft 4 passing through such gaps. This may possibly allow the paint to remain in the valve chamber 5 in sliding contact with the circumferential surface of the cylindrical magnet MS.
Furthermore, if the paint intruding between the circumferential surface of the cylindrical magnet MS and the inner wall of the valve chamber 5 in sliding contact with each other remains and solidifies there, it is no more possible to move the cylindrical magnet MS with the magnetic force of the cylindrical magnet ML, thereby hindering the valve ON-OFF operation.